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 mirror silvering
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By: Anonymous: zeena () on Thursday, July 19 2007 @ 02:50 PM PDT (Read 7226 times)  
Anonymous: zeena

i have a large round mirror i got at a tag sale that has lost most of it's silvering. i want to loose the rest of it as it fits perfectly on top of a round table i have. i called a few glass and mirror places to ask them how to get rid of it and got some very weird answers . . . acid nail polish remover razor blades . . . so you see i need someone with great knowledge to give me the answer i need to get this stuff off.

By: Windell (offline) on Thursday, July 19 2007 @ 03:28 PM PDT  

This is actually a very common problem that astronomers face, because their telescopes need to have the silvered coating (almost always aluminum, actually) replaced every so often. And, the new coating can't be applied until the old one is removed. Razor blades can remove the silvering and the backing paint that sometimes covers it. However, it *will* mar the glass underneath, leaving faint but visible scratch marks or trails. I suggest *not* doing that. Nail polish remover, or much better, pure acetone, will remove many types of backing paint. If there is paint over your mirror coating, you will need to remove that before attaching the metal layer. Acetone cannot affect the metal layer itself, but it can help you get closer to it. Once you have the metal layer exposed, you can start to think like the astronomers do. The astronomers do use acid to remove the coating (See here), but if you don't want to work directly with acid you can use ferric chloride to do the work. Wear nitrile gloves-- it's nasty. Finally, you might want to try and find a stained glass shop. Apparently, there are such things as mirror stripping kits, designed to help you do exactly this. -Windell.

Windell H. Oskay

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By: Anonymous: zeena () on Thursday, July 19 2007 @ 06:54 PM PDT  
Anonymous: zeena

well windell i believe you make a good point in my heading to the stained glass shop . . . checked out the other stuff you attached and i think i might be a little over my head with that stuff. never thought of the stained glass shop though . . . and i know just where one is.

thanks buddy.


By: Anonymous: Dave () on Monday, July 23 2007 @ 08:34 AM PDT  
Anonymous: Dave

I'll mostly second what Windell said. Mostly.

The first thing to do would be to determine what the metal used as a silvering material is. Old mirrors usually used real Silver, but more modern mirrors probably use Aluminum (due to cost issues). One way to tell is that Silver may have a pinkish tint to it.

Next, having identified the silvering material, the next step is to remove it. As Windell says, physically abrading it may damage the glass. Steel (e.g., razor blades) has a Mohs hardness of between about 5.5 and 7. Glass also has a Mohs hardness of about the same. One of the problems is that there is no such thing as glass. The term glass covers a WIDE variety of transparent materials, all the way from Soda-Lime glass up through Borosilicate glass up to fused Quartz (which has a Mohs hardness of 7.0, and is a real pain to machine/abrade). In any case, most mirrors are made from an inexpensive "glass", which usually has a rather low hardness, and which may be scratched (at least, somewhat) by some of the harder steels (Oh, by the way, there is no such thing as steel. The term steel refers to a whole variety of Iron alloys with varying hardnesses and other mechanical properties.).

One way of removing the silvering is to chemically etch it. However, if the back has been painted or covered with a protective layer (in order to prevent the accidental etching of the silvering over time), this must first be removed. Acetone is an excellent solvent for quite a few materials, and is readily available (usually in the paint section of most large department stores). However, it's also a liver carcinogen, so definitely try to minimize your exposure to it. It's also highly volatile, so don't use it in an enclosed space, and extremely flammable, so don't use it around a source of ignition.

Once the protective covering (if there was one) has been removed, then the silvering may be attacked. If the material is pure Silver, it can be attacked by quite a few acids. However, the idea here is that the resulting Silver compound formed via the action of the acid should be soluble so that it can be washed away. One of the most common readily available acids is Hydrochloric Acid. However, the resulting Silver Chloride is only very slightly soluble. A much better choice would be Silver Nitrate, but this would require etching the silvering with Nitric Acid, which may be hard to obtain locally [1].

[1] Plus, Nitric Acid has an, umm, interesting effect if you happen to spill it on your fingers. It reacts with some of the proteins to form a bright yellow compound which can not be washed off. If you spill it on your fingers, you will have yellow stained fingers for the next six weeks or so (short of chopping your fingers off!).

If the silvering is Aluminum, then the Hydrochloric Acid should do the trick (I think), since Aluminum Chloride is soluble. The trick is that Aluminum forms a tough, transparent layer of Aluminum Oxide when exposed to the air, which prevents most other things from getting to the metallic Aluminum, thus possibly preventing the reaction. And, there aren't a lot of things which attack the Aluminum Oxide layer, excepting possible a strong Sodium Hydroxide solution. However, the problem with this is that Sodium Hydroxide also attacks glass.

In any case, all of these chemicals require extreme care, since quite a few of them will attack human flesh quite readily. The best solution may be to find a commercial (de/re)silvering kit.

Note that not all stained glass shops carry mirror kits [2], but they should be able to either order them or point you to a source.

[2] The significant other works in a stained glass shop, but they tend to stay away from the more exotic chemical stuff.



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